An organic light-emitting diode device, also called an OLED device, commonly includes a substrate, an anode, a hole-transporting layer (HTL), light-emitting layer (LEL), an electron-transporting layer (ETL), and a cathode. OLED devices are attractive because of their low drive voltage, high luminance, wide viewing angle, and capability for full color flat emission displays and for other applications. Tang et al. describe this multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
An OLED device can emit different colors, such as red, green, blue, or white, depending on the emitting property of its LEL. There is a great demand for white-light OLED (or white OLED) devices where there is substantial emission in the red, green, and blue portions of the spectrum and generally have 1931 Commission Internationale d'Eclairage (CIE) chromaticity coordinates, (CIEx, CIEy), of about (0.33, 0.33). White OLED devices can be used for solid-state lighting sources and for full color displays. As a solid-state lighting source, a white OLED device generally needs a capability to have very bright emission. As an emitting pixel in a full color display array, a white OLED device also needs a high brightness because the final color as perceived by the viewer is dictated by a corresponding color filter element which reduces the initial emission intensity by about two third (⅔). In order to achieve the same viewing brightness as that of a full color display using red, green, and blue OLEDs as emitting elements, the white OLED has to be driven under higher current density. However, an OLED device will rapidly degrade when driven at high current density. Although white OLED devices have already achieved high brightness as reported by J. Shi et al. in U.S. Pat. No. 5,683,823, Sato et al. in JP 07-142169, Kido et al. in “White light-emitting organic electroluminescent devices using the poly(N-vinylcarbazole) emitter layer doped with three fluorescent dyes”, Applied Physics Letters, 64, 815 (1994), Deshpande et al. in “White light-emitting organic electroluminescent devices based on interlayer sequential energy transfer”, Applied Physics Letters, 75, 888 (1999), and Tokito et al. in “High-efficiency white phosphorescent organic light-emitting devices with greenish-blue and red-emitting layers”, Applied Physics Letters, 83, 2459 (2003), further improvement is still needed.
Recently, a new kind of OLED structure called tandem OLED (or stacked OLED, or cascaded OLED), which is fabricated by stacking several individual OLEDs vertically and driven by a single power source, has also been proposed or fabricated by Jones et al. in U.S. Pat. No. 6,337,492, Tanaka et al. in U.S. Pat. No. 6,107,734, Kido et al. in JP Patent Publication 2003045676A and in U.S. Patent Publication 2003/0189401 A1, and Liao et al. in U.S. Pat. No. 6,717,358 and U.S. Patent Application Publication 2003/0170491 A1. For example, Tanaka et al., U.S. Pat. No. 6,107,734, demonstrate a 3-EL-unit tandem OLED using In—Zn—O (IZO) films or Mg:Ag/IZO films as intermediate connectors and achieved a luminous efficiency of 10.1 cd/A from pure tris(8-hydroxyquinoline)aluminum emitting layers. Matsumoto et al., “Multiphoton Organic EL Device having Charge Generation Layer”, SID 03 Digest, 979 (2003), fabricate 3-EL-unit tandem OLEDs using In—Sn—O (ITO) films or V2O5 films as intermediate connectors and achieved a luminous efficiency of up to 48 cd/A from fluorescent dye doped emitting layers. Liao et al., “High-efficiency tandem organic light-emitting diodes”, Applied Physics Letters, 84, 167 (2004), demonstrate a 3-EL-unit tandem OLED using doped organic “p-n” junction layers as intermediate connectors and achieved a luminous efficiency of 136 cd/A from phosphorescent dye doped emitting layers.
The performance of white OLED devices can be improved by using the aforementioned tandem OLED device structures. For example, Matsumoto et al., in FIG. 8 of “Multiphoton Organic EL Device having Charge Generation Layer”, SID 03 Digest, 979 (2003), demonstrate that a tandem white OLED device can be constructed by connecting a greenish blue EL unit and an orange EL unit in the device, and white light emission can be achieved by driving this device with a single power source. Liao et al. in FIG. 7 of U.S. Patent Application Publication 2003/0170491 A1 propose to construct a tandem white OLED by connecting a red EL unit, a green EL unit, and a blue EL unit in series in the device. When the tandem white OLED is driven by a single power source, white light emission can be formed by spectral combination from the red, green, and blue EL units.
However, the prior art has only taught how to construct a tandem white OLED device using multiple EL units, each unit having a different primary color emission, i.e., having a red, green, or a blue color emission from each of the EL units. With this configuration, the white color will be changing with time because each EL unit has different degradation behavior during operation. Moreover, device fabrication will be more complicated and more time consuming because each EL unit has a different layer structure.